What is Amino Acid Residue?

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  • 0:05 Amino Acids and Proteins
  • 1:10 Residues and Protein Structure
  • 2:11 Disulfide Bridges
  • 2:34 Denaturation
  • 3:34 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Amino acid residue is the part of an amino acid that makes it unique from all the others. Its features, such as how it interacts with water, help guide the structure of a finished protein.

Amino Acids and Proteins

When you hear the word residue, you normally think of something left over. When you wash a baking pan, for example, it sometimes has some sticky cooking spray residue. But when talking about amino acids, a residue is a specific and unique part of the amino acid.

Let's start by talking about amino acids, the building blocks that link together to form proteins. Proteins perform a large chunk of the important jobs a cell needs to do. They make up your muscles, hair and nails; they make chemical reactions happen; and they digest your food.

An amino acid is made up of a few different parts, connected together. The parts of an amino acid are an amine group, a carboxylic acid group, and the residue. The amine and carboxylic acid groups give the name 'amino acid,' and these two parts are identical to those of other amino acids.

The residue is the part that is unique among each of the 20 amino acids. Think of the generic definition of residue as something leftover. An amino acid residue is what's left over when you take away all the identical parts of the amino acid.

Structure of an amino acid
Amino acid

Residues and Protein Structure

Amino acid residues are important because they are the unique portion of an amino acid. They are the part that gives the amino acid its personality, so to speak. Some amino acid residues are polar, meaning they have a charge. These polar amino acid residues are hydrophilic, meaning they interact with water ('hydro-' for 'water', and '-philic' for 'loving'). Other amino acids are nonpolar, which means without a charge. Nonpolar amino acid residues are hydrophobic, which means they don't like to interact with water.

When amino acids are lined up to form a protein, they'll arrange themselves so that hydrophilic residues are exposed to water and hydrophobic residues are hidden from water. This can cause the protein to form into analpha-helix, which is a coiled up shape, or to form into a beta pleated sheet, which is a zig-zag shape. The shape a protein takes is incredibly important for its function. Think about it - if a key has the wrong shape, it can't open a lock, right? The same is true for a protein and its function.

Alpha helices of protein shown in red
Alpha helix

Disulfide Bridges

Amino acid residues can also link pieces of a protein together. One particular amino acid, cysteine, has a sulfur atom. Sulfur atoms on two separate cysteine residues can bond with each other, forming a disulfide bridge, which links together parts of a protein that would otherwise be far apart. This is another way proteins get their specific shapes.

Disulfide bridges link together parts of a protein that would otherwise be far apart

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